Spring 2011 Midterm Review Answers

Report
Spring 2011
Midterm Review Answers
CP PHYSICS
MS. MORRISON
1. How are positive ions formed? How are negative
ions formed?
 Positive ions are formed by atoms that lose electrons
 Negative ions are formed by atoms that gain
electrons
2. What is the fundamental rule regarding charge
interactions?
 Like charges repel and opposite charges attract.
3. Can charges be created or destroyed?
 NO – they can only be separated. This is done when
electrons are transferred from one object to another
resulting in a negatively charged object and a
positively charged object.
4. How are conductors different from insulators?
 Conductors have electrons that can move easily from
one atom the next while insulators hold their
electrons tightly, so that they do not move.
5. Explain how charges can be moved by:
 a. Conduction: a neutral body is charged when touched by a
charged object, it takes on the same charge as the charged
object, is permanent, repels
 b. Induction: a neutral object becomes temporarily charged
when a charged object is brought near it, takes on the opposite
charge, attracts
 c. Friction: two neutral objects become charged when they
are rubbed together and electrons are transferred from one
object to the other, results in one object having a positive
charge and the other object having a negative charge so that
they become attracted to each other
 d. Grounding: a charged object’s excess charge moves from it
to through to the ground when a pathway from it to the
ground is provided
6. What happens to the electrical force experienced by two
charged particles separated some distance if:
 a. One of the charges doubles: 2 F
 b. The distance is doubled: ¼ F
 c. One of the charges doubles and the distance is
doubled: ½ F
7. Two identical charges of 3.8 x 10-6 C are separated by a distance of
0.025 m. What electrical force do they exert on each other?
Q1 = Q2 = 3.8 x 10-6 C
d = 0.025 m
k = 9 x 109 Nm2/C2
F=?
Fel = kQ1Q2
d2
= (9 x 109)(3.8 x 10-6)(3.8 x 10-6)
(0.025)2
Fel = 207.94 N
8. How far apart are two negative charges if they experience a force of 5.6 N on
each other? One has a charge of -1.5 x 10-5 C, and the other has a charge of -7.2 x
10-4 C.
Fel = 5.6 N
Q1 = -1.5 x 10-5 C
Q2 = -7.2 x 10-4 C
k = 9 x 109 Nm2/C2
d=?
Fel = kQ1Q2
d2
5.6 = (9 x 109)(-1.5 x 10-5)(-7.2 x 10-4)
d2
(5.6)d2 = (9 x 109)(-1.5 x 10-5)(-7.2 x 10-4)
5.6
5.6
d2 = 17.36
d = 4.17 m
9. Draw the electric fields for the following:
a. Weak positive charge
b. Strong negative charge
9. Draw electric fields for the following:
c. Two positive charges
d. A negative charge and a
positive charge
10. What are the relationships between current and voltage
and resistance according to Ohm’s Law?
 Current and voltage are directly proportional to each
other. If the voltage increases, then the current will
also increase and vice versa. (Assume resistance is
constant.)
 Current and resistance are inversely proportional to
each other. If the resistance increases, then the
current will decrease and vice versa. (Assume
voltage is constant.)
11. Do you buy electrons from the power company?
 No, you buy energy from the power company. The
electrons are already in your electrical devices and
home wiring, and they flow because of the energy
received from the power company.
12. What are two safety devices used in circuits? How do
they work?
 Fuses: small tube with metal ends and wire through
middle, wire melts if the current increases beyond a
specific value, must be replaced if used
 Circuit breakers: breaker pops open and in turns
opens up the circuit when the current exceeds a
specific value, can be reused, the breaker simply
needs to be reset
13. What is the difference between direct current and
alternating current?
 Direct current is current that flows only in one
direction.
 Alternating current is current that moves back and
forth in two directions – results from the way it is
produced.
14. What is a source of direct current? Of alternating
current?
 Batteries provide direct current.
 Generators (including power companies) provide
alternating current.
15. What is the current through an electrical device that is plugged
into a 120 V outlet, if it experiences a resistance of 540 Ω?
V = 120 V
R = 540 Ω
I=?
V = IR
120 = I (540)
540
540
I = 0.22 A
16. What is the voltage across a device that provides 1200 Ω of
resistance and has a current of 0.40 A through it?
R = 1200 Ω
I = 0.40 A
V=?
V = IR
= (0.40)(1200)
V = 480 V
17. What is the resistance of an electric frying pan which draws 5.8
amps when connected to a 120 V circuit?
I = 5.8 A
V = 120 V
R=?
V = IR
120 = 5.8 R
5.8 5.8
R = 20.69 Ω
18. What is the power of the frying pan in #17?
I = 5.8 A
V = 120 V
P=?
P = VI
= (120)(5.8)
P = 696 W
19. An electrical appliance uses 6.0 kWh in a month. If the power company
charges $0.08/kWh, what is the cost to use this electrical appliance?
E = 6.0 kWh
rate = $0.08/kWh
cost = ?
cost = E x rate
= (6.0)($0.08)
cost = $0.48
20. What is a series circuit?
 A circuit in which there is only one pathway for
current to move. It does not matter how many
electrical devices (resistances) are plugged into the
circuit, there is only one way for the current to flow –
it will flow through all the devices in the circuit.
21. What is a parallel circuit?
 A circuit in which there are two or more pathways for
current to move.
22. If one light goes out in a series circuit, do the
remaining lights go out?
 Yes, the circuit is essentially open, so no current can
flow through it. This interrupts the flow of current
through every other device in the circuit.
23. If one light goes out in a parallel circuit, do the
remaining lights go out?
 No, the remaining lights will stay on if they are in
different branches of the circuit. This is because the
different branches provide alternate pathways for the
current to flow.
24. What happens to the equivalent (total) resistance in a
series circuit when you add another resistor to it?
 The total resistance will increase every time another
resistor is added to the circuit. This is because the
total resistance is calculated by adding up all the
individual resistances in the circuit.
25. What happens to the equivalent resistance in a parallel
circuit when you add another resistor to it?
 The total resistance decreases every time another
resistor is added into the circuit in parallel. This is
because the total resistance is less than the lowest
resistor in the circuit. (This is also why circuit
breakers are tripped when too many resistors are
plugged into the circuit – think too many hair
dryers.)
26. What does an ammeter measure? How is it connected
into a circuit?
 An ammeter measures the current flow through the
circuit.
 It is connected in series, because the current must
flow through it to get an accurate measurement.
27. What does a voltmeter measure? How is it connected
into a circuit?
 A voltmeter measures the voltage across an electrical
device.
 It is connected into a circuit in parallel because it
provides another pathway though which current can
flow. Remember – voltage is also known as potential
difference, so to get a reading of the potential
difference, the meter must be connected into the
circuit before and after the electrical device.
28. If there are three lamps connected in series, how many
paths can the current take?
 One – a series circuit has only one pathway through
which the current can flow.
29. If there are three lamps connected in parallel, how
many paths can the current take?
 If each lamp is in its own separate branch of the
circuit, then the current has three pathways it can
take through the circuit.
30. Determine the equivalent resistance for the following circuits:
R = 15 Ω
R = 11 Ω
31. Calculate the values represented in the missing blanks for the following circuit
diagrams. Also, identify whether the circuit is a series or parallel circuit.
Series
V1 = 40 V, IT = 2 A, R2 = 30 Ω
Series
RT = 110 Ω, V1 = 40 V, V2 = 120 V,
V3 = 60 V, VT = 220 V
31. Calculate the values represented in the missing blanks for the following circuit
diagrams. Also, identify whether the circuit is a series or parallel circuit.
Parallel
V1 = 40 V, V2 = 40 V, I1 = 2 A,
I2 = 2 A, R2 = 20 Ω, RT = 10 Ω
Parallel
RT = 15 Ω, VT = 60 V
32. How are magnetic poles similar to electric charges?
 Like charges/poles repel each other.
 Opposite charges/poles attract each other.
 Both exert forces that can act over a distance – the
charges/poles do not have to be touching each other
for the force to act between them.
33. What is the major difference between magnetic poles
and electric charges?
 Electric charges can be separated from each other –
electrons can be removed so that there is a negatively
charged object and a positively charged object.
 Magnetic poles cannot be separated – you cannot
separate the north pole from the south pole. If you
break the magnet in half, all you get is two smaller
magnets each with a north and south pole.
34. How is the magnetic field oriented around a magnet?
 The magnetic field lines leave the north pole and
curve around the magnet and enter the south pole.
35. Where is the magnet field the strongest?
 The magnetic field of a magnet is strongest at its
poles.
36. What is the smallest magnet?
 The smallest magnet in the universe is an electron.
This is because magnetic fields are created by
moving charges, and electrons spin on their axes just
like Earth spins on its axis.
37. What happens to the magnetic field around a currentcarrying wire if the current through it is reversed?
 If the current flow is reversed, then the direction of
the magnetic field will also be reversed.
38. How does an electric motor work?
 A coil of wire is placed inside a permanent magnet so
that when electricity goes through the wire it
becomes an electromagnet and interacts with the
permanent magnet’s magnetic field.
 The wire then spins and every 180o the current
reverses direction so that the poles of the
electromagnet flip so that the wire will keep spinning
and spinning.
 Motors convert electrical energy into mechanical
energy.
39. What are two ways to generate current in a wire?
 Move a wire in and out of a magnetic field (magnet).
 Move a magnet in and out of a coil of wire.
40. How does an electric generator work?
 A coil of wire is located inside permanent magnet –
just like a motor.
 When you turn the coil of wire (steam, wind, water),
then electricity is generated because the wire is
moving within the permanent magnet’s magnetic
field.
 The electricity generated is alternating current.
 Generators convert mechanical energy into electrical
energy – opposite of a motor.
41. What three factors does the strength of an
electromagnet depend on?
 The number of loops in the wire – more loops =
stronger electromagnet
 The amount of current flow through the coil of wire
– more current = stronger electromagnet
 Presence of an iron core – if iron core is present,
then the electromagnet will be stronger
42. What three factors does the voltage (and current)
produced by a generator depend on?
 The number of loops in the wire – more loops =
more voltage generated
 The strength of the permanent magnet’s magnetic
field – stronger magnet = more voltage generated
 The speed of rotation of the coil of wire – the faster
the coil of wire is rotated = more voltage generated

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